1. Field of the Invention
This invention relates to gel type materials. More particularly, this invention relates to a method for transporting cured gels derived from organic or organosiloxane polymer compositions. The gel is transported as an integral, homogeneous mass suitable for use as a coating or encapsulating material.
2. Description of the Prior Art
Organic and organosilicon gels are well known classes of materials. These products are typically lightly crosslinked, unfilled, relatively weak and deformable polymers that do not flow under gravity. The low degree of crosslinking often results in a surface that can be described as sticky or "tacky" to the touch, a property responsible for the excellent adhesion exhibited by the gel to a wide variety of substrates. Gels can also be characterized by their ability to absorb mechanical stresses, to maintain their consistency over a wide temperature range, and to "self-heal" when punctured using a knife or a laboratory spatula.
The elongation value of a typical gel is at least 200 percent.
The unique combination of properties exhibited by gels, particularly organosilicon gels, make them desirable for use as protective coatings and encapsulants on substrates that are exposed to a wide range of temperatures, harsh environments and/or mechanical stresses. Because many gels are excellent electrical insulators, they are used to encapsulate delicate electronic devices such as transistors and integrated circuits and as filling materials for electrical junction boxes and other enclosures containing uninsulated electrical conductors.
Gels are typically prepared by reacting a liquid organic or organosilicon monomer containing functional groups such as vinyl or hydroxyl. Depending upon the type of functional group, the monomer is reacted with free radicals generated by an initiator such as an organic peroxide or ultraviolet light, or with a curing agent containing a plurality of functional groups, such as silicon-bonded hydrogen atoms, that react with the functional groups present on the monomer. To achieve the lightly crosslinked structure that characterizes a gel, when a curing agent is used, a stoichiometric excess of one of the two types of functional groups that react to cure the gel is typically present. The concentration of functional groups is selected to achieve a cured product of the desired consistency and hardness. The hardness of both silicone and organic gels is typically too low to be measured using the Shore A or Shore D durometer scales. The hardness values of some gels can be measured on the Shore 00 scale. A convenient method for defining the hardness of gels is the depth in millimeters to which a probe of specified weight will penetrate the surface of the gel. This value is generally from 1 to about 30 millimeters, preferably from 1 to about 20 millimeters, for a 6.4 mm.-diameter probe weighing 19.5 grams.
In accordance with prior art methods a mixture of the liquid ingredients that react to form the gel is placed on the substrate to be coated or encapsulated. Depending upon the functional groups present on the ingredients, the mixture will cure under ambient conditions, upon heating at temperatures of from about 50 .degree. to about 200.degree. C., or when irradiated with ultra-violet light. Depending upon the type of curing reaction and any inhibitors present, heating periods of from several minutes up to an hour or more may be required to cure the composition.
Curing a gel at the location where it is to be applied has at least two disadvantages. Firstly, if the gel is prepared by a platinum-catalyzed hydrosilation reaction or other type of curing reaction requiring a two-part composition to avoid premature curing, specialized equipment to meter, blend and dispense two liquid materials in a specified weight or volume ratio is needed to achieve proper curing and the desired physical properties. The equipment and expertise required to prepare satisfactory gels in a reproducible manner may not be available at the location where the ingredients used to prepare the gel are blended and cured.
Secondly, the use of a liquid curable composition requires providing retainers to maintain the composition at the desired location at least until it cures sufficiently to be non-flowable under the influence of gravity. The need for retainers can present a problem if the gel is used to fill an enclosure such as a electrical junction box, terminal block or connector that contains openings through which wires, cables or other conductors are inserted after the gel is cured.
The tackiness and friability exhibited by cured gels makes it difficult to remove a portion of cured gel from a container in which it is prepared and apply it on the surface of a substrate as a coherent coating in the same manner as one would apply a paint or a grease.
U.S. Pat. No. 4,750,962, which issued to Haygood et al. on June 14, 1988 teaches a method for picking up a cured gel and placing it at a desired location. This patent teaches that gels are difficult to dispense in an automated manner due to their very tacky surface, which causes the gel to adhere to the interior of the dispensing equipment. The method taught in the patent utilizes the relatively weak peel strength of gels by placing a strip or slab of cured gel on a sheet of release paper. One of the disclosed methods for removing the gel from the release paper involves inserting the edge of a blade between the gel and the paper, utilizing the adhesion of the gel to the blade to lift the gel off the paper and transporting the blade containing the adhered gel to the substrate where the gel is to be applied. A second method involves cutting and removing a portion from a layer of gel located on a release paper using a device equipped with means for cutting the gel and applying either vacuum or positive air pressure to the portion of gel cut from the layer. Vacuum is applied to lift the cut portion of gel and retain it in position while the device is moved to the location where the gel is to be dispensed. Positive air pressure is then used to eject the portion of cured gel from the device.
An objective of this invention is to provide a method for transferring a portion of cured silicone or organic gel from the container in which it is cured to the location where it will be applied without fragmenting the gel or otherwise adversely affecting its structure, homogeneity, physical properties or appearance.